1. Field of the Invention
The present invention relates to a process for reparing aromatic polyamides. More specifically, the nvention relates to a process for economically and fficiently preparing aromatic polyamides.
2. Prior Art
Aliphatic polyamides (nylons) such as nylon 6, nylon 66 and nylon 610 have heretofore been widely used as engineering plastics in various fields. The aliphatic polyamides can be favorably molded but have low glass transition temperatures (e.g., nylon 66 has a glass transition temperature of generally about 60.degree. C.) and, hence, exhibit insufficient heat resisting property.
In order to solve these problems, the present inventors have proposed an aromatic polyamide including (a) a dicarboxylic acid component unit comprising 60 to 100 mol % of a component unit derived from a terephthalic acid, and 0 to 40 mol % of a component unit derived from an aromatic dicarboxylic acid other than terephthalic acid, and (b) a component unit derived from an alkylene diamine, the aromatic polyamide having an intrinsic viscosity [.eta.] of from 0.5 to 3.0 dl/g as measured in the concentrated sulfuric acid at 30.degree. C. (see Japanese Unexamined Patent Publication (Kokai) No. 53536/1984). The above aromatic polyamides exhibit excellent heat resisting property and mechanical properties, but have high melting points. When they are prepared according to the traditional method of preparing aliphatic polyamides, therefore, the aromatic polyamide is, in some cases, decomposed in the step of production, and the hue of the aromatic polyamide is lowered.
In order to prepare the aromatic polyamides, therefore, the present inventors have proposed a method according to which a polyamide having a low molecular weight (low-degree polyamide) is, first, prepared from a dicarboxylic acid and a diamine and, then, the low-degree polyamide (the polyamide obtained by the polymerization with low polymerization degree) is further polymerized so that the resulting polyamide possess a desired molecular weight (see Japanese Unexamined Patent Publication (Kokai) No. 196625/1988).
In order to polymerize the low-degree polyamide so that the resulting polyamide possess a desired molecular weight, furthermore, Japanese Unexamined Patent Publications (Kokai) Nos. 228693/1995 and 311198/1996 disclose methods in which a solid low-degree condensation product (the product obtained by the condensation with low condensation degree) is solid-phase polymerized in vacuum or in a carrier gas stream, followed by the melt polymerization.
However, the present inventors have discovered the fact that especially when an aromatic polyamide containing a terephthalic acid component further contains a component unit derived from a cyclic dicarboxylic acid, the following problem occurs in the solid-phase polymerization. Namely, the problem is that a low-degree condensation product give agglomerates probably due to the water of condensation formed during the solid-phase polymerization, and according to the conventional preparation method, therefore, the solid-phase polymerization does not efficiently proceed. It has therefore been urged to provide a method capable of more efficiently preparing an aromatic polyamide containing a cyclic dicarboxylic acid component unit. The solid-phase polymerization does not efficiently proceed, either, when the aromatic polyamide contains a component unit derived from a long-chain diamine having 4 to 25 carbon atoms. It has therefore been urged to provide a method capable of efficiently preparing the aromatic polyamides containing such a component unit.